This invention generally relates to an improved polygon attachment for coupling different members. In particular, the invention relates to a coupling for connecting an impeller to a pinion or drive shaft.
Polygon shaft couplings are generally known. For example, this type of shaft coupling is described in U.S. Pat. No. 4,294,562, which is incorporated by reference herein. Typically, in such couplings, a shaft and a hub have cross sections fitting into each other, and these cross sections deviate from a circular pattern and have shapes that are convex on all sides and essentially elliptical, triangular or quadratic. The polygon coupling provides several advantages over more conventional splined connections by eliminating problems such as wear on the splines, notch effects and higher torsional stresses.
A polygon coupling may be used in a centrifugal compressor design, as shown in U.S. Pat. No. 5,482,437, which is incorporated by reference herein. For example, an impeller of a centrifugal compressor may be operatively connected to a centrifugal compressor driving shaft or pinion. In such a design, the compressor impeller includes a polygonally dimensioned male hub portion, and the compressor pinion includes a suitably dimensioned bore which receives the male hub portion of the compressor impeller. The polygon attachment must transmit torque from the pinion to the impeller, while maintaining zero relative motion between the impeller and the pinion, and permit easy assembly and disassembly that consistently returns the two parts to the original relative positions. The maintenance of position is critical to assure that the assembly retains its dynamic balance. Although this type of polygon coupling has been used with success in centrifugal compressor design, this polygon coupling is difficult to produce because of the exacting tolerances required.
As shown in FIG. 7, the polygon must consistently xe2x80x9clock-upxe2x80x9d at the same position to maintain the needed level of rotor balance. Any relative movement leads to unacceptable levels of vibration during compressor operation. To assure the needed consistency, the mating parts must be machined to very exacting and expensive tolerances. Typically, the profile tolerances on are on the order of +/xe2x88x920.00015 inches. Such tolerances are difficult to achieve and are difficult to validate by inspection. Therefore, it is desirable to provide an improved design which retains the advantages of the polygon attachment, but alleviates the manufacturing and inspection difficulties.
According to the present invention a coupling for attaching two members is provided. The invention includes a coupling comprising a first coupling member including a tapered bore having a polygonal shaped cross-section. The coupling further includes a second coupling member including a tapered plug having a polygonal shaped cross-section. The plug is configured to be received into the bore of the first coupling member. A fastener may be provided for securing the second coupling member to the first coupling member, the fastener being adapted to be inserted into a passage in the plug of the second coupling member. The plug of the second coupling member is split so that when the fastener is inserted into the passage the plug expands against the bore thereby creating an interference fit between the first and second coupling members.
The coupling may further include a recess positioned in a base of the bore of the first coupling member for receiving the end of the fastener. The fastener may include an end portion having a cross-sectional area greater than the crosssectional area of the bore of the second coupling member. The bore of the second coupling member may include a region of reduced cross-sectional area adapted to surround the end portion of the fastener. The end portion of the fastener may include a threaded portion to be secured in the recess located in the base of the bore.
In an alternative embodiment an impeller and pinion assembly is provided. The assembly may include a pinion having a tapered bore with a polygonal shaped cross-section. The impeller may include a tapered plug having a polygonal shaped cross-section, the plug being configured to be received into the bore of the pinion. A fastener may be provided for securing the impeller to the pinion, the fastener adapted to be inserted into a passage in the plug of the impeller. The plug of the impeller is split so that when the fastener is inserted into the passage the plug expands against the side wall of the bore thereby creating an interference fit between the pin ion and the impeller.
The assembly may further include a recess positioned in the base of the bore of the first coupling member for holding the fastener. Preferably, the passage in the plug is drilled. The fastener may include an end portion having a region with a cross-sectional area substantially greater than the cross sectional are of a shank portion of the fastener. The end portion may further include a threaded portion to be positioned in the recess.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are only intended to provide a further explanation of the present invention, as claimed. The foregoing and additional objects and advantages of the invention will become more apparent as the following detailed description is read in conjunction with the accompanying drawings.